1. Field of the Invention
This invention relates to a novel method of producing a precursor pitch as a starting material for the production of carbon fibers.
2. Related Art Statement
The production of carbon fibers is roughly classified into a method of firing synthetic fiber such as polyacrylonitrile fiber or the like and a method of spinning a tar pitch as a starting material and carbonizing the resulting fiber. Among them, the former method has drawbacks that the cost of the starting material is high and the carbonization yield is low. On the other hand, in the latter method, there is no problem on the cost and carbonization yield, but it is necessary to prepare a precursor pitch for the production of carbon fibers from the starting pitch. In the preparation of such a precursor pitch, it is required to remove insoluble solid matter from the starting pitch. Further, since mesophase spherules (adversely affecting the spinnability and the strength of carbon fiber) are produced during the heating, if it is intended to prevent the occurrence of mesophase spherule, the low molecular weight component can not be removed sufficiently and consequently the infusibility is unfavorably degraded. In order to improve the infusibility, there has been proposed a method of hydrogenating the starting pitch with a hydrogen gas under a high pressure or with a specific hydrogen donor solvent in Japanese Patent Application Publication No. 45-28018, but this method is not yet suitable for an industrially practical use.
Furthermore, a method of removing a solvent insoluble matter from coal tar or the like has been proposed in Japanese Patent Application Publication No. 49-26481 and Japanese Patent laid open No. 52-28501. In this case, quinoline insoluble matter contained in the starting coal tar (adversely affecting the quality of objective needle pitch coke) is extracted and separated with a petroleum solvent, e.g. an aliphatic solvent, which is entirely different from the object of the invention as mentioned below.
In Japanese Patent laid open No. 57-159885, there is disclosed a method wherein a heavy coal oil is added with a ketone solvent to remove insoluble matter and then the resulting pitch is subjected to a heat treatment. In this case, the particular ketone solvent must be used, and further a nitration solvent must be added in the heat treatment.
In Japanese Patent Application Publication No. 55-144087, there is disclosed a method of extracting a substance having particular properties with a solvent from soluble matter obtained after the removal of quinoline insoluble matter from the pitch. In this case, however, the yield is low and the production step is complicated. Further, the substance having particular properties is a starting material for optically antisotropic pitch, which is essentially different from optically isotropic pitch aimed at by the present invention.